I've just replaced mine for the ZHP gear knob which I bought from one of the other members. It seems to be the recommended OEM part on here. You can pick it up brand new from the dealer for about £45. It's a bit shorter than the original and a lot comfier to use. Here's the link to the thread I bought mine from, I posted a few links to the part price and number a bit down the thread. http://www.z4-forum.com/forum/viewtopic.php?f=7&t=12005
Let this distinctive hand polished, high gloss chrome finished shift knob enhance your BMW driving experience. Available in 5-speed or 6-peed patterns.
5-Speed Pattern (PN: 25 16 0 411 927) $93.60
6-Speed Pattern (PN: 25 16 0 411 928) $93.60